DESCRIPTION The broad and long-term goal of the proposal is to study structural changes and motions of individual bio-macromolecules such as proteins, DNA, RNA and their complexes in equilibrium, during folding, upon ligand binding, and during catalysis. The single molecule measurements can yield information on the distributions and time trajectories of the structural changes and motion of molecules that can not be revealed by ensembles experiments, and thus provide new insights into our fundamental understanding of the bio-macromolecules. Such a molecular understanding is key to our understanding of the biological systems and the origin of various diseases and can speed up the development of cures to the diseases and the designing of new biological material. Specifically, new tools to study individual bio-macromolecules using fluorescence microscopy are being developed and being applied to problems including protein folding, RNA folding and ribosomal translation. In the folding experiments, the protein or RNA will be labeled with two identical fluorescence molecules or a pair of fluorescence donor and acceptor. The conformation changes of the protein or RNA during folding will lead to changes in the fluorescence. By measuring the fluorescence time-traces from the individual protein or RNA molecules under various conditions, their structures and folding kinetics can be explored. In the ribosomal translation experiment, two different components in the ribosome complex will be labeled with a pair of fluorescence donor and acceptor. The relative motion between the two components during the translation will lead to changes in the fluorescence energy transfer efficiency between the donor and the acceptor. By varying the labeling scheme and measuring the fluorescence during the translation, the molecular details of the ribosomal translation process can be obtained.